1. Field of the Invention
The present invention relates to an objective lens detachably mounted on an image-forming portion for forming an image from light received by the objective lens, and to an optical measuring device including the objective lens.
2. Description of Related Art
An infinity-correction optical system for forming an image by using an objective lens and an image-forming lens (tube lens) has been conventionally so flexible in terms of layout of optical systems as to allow an illumination optical system to be included therein because light between the objective lens and the tube lens is parallel. Thus, such an infinity-correction optical system has been applied to various optical machines, a representative example of which a metallographic microscope excellent in resolution and flatness.
In microscope objective lens(es) for forming such an infinity-correction optical system, numerical aperture (hereinafter abbreviated as NA), as well as wavelength to be used, is an important parameter for determining resolution and focal depth.
Many manufacturers of optical machines provide an objective lens having standard resolution and standard focal depth, an objective lens having high resolution, small focal depth and high NA, and an objective lens having reduced NA and relatively large focal depth for one magnification so as to meet observation-related demands from users.
According to a known arrangement, without providing plural objective lenses that are mutually different in NA, NA is changeable (see, for instance, Document 1: JP-A-10-206741).
According to Document 1, a position of a rear focal point of an objective lens is moved by a relay lens toward an eyepiece so as to form an optical conjugate point, and a diaphragm is disposed at the conjugate point. The diaphragm is adapted to change NA of the objective lens.
However, in the above-described arrangement where plural objective lenses are provided, the objective lenses need to be replaced in accordance with types of objects, which may cause operation efficiency to be lowered.
In addition, when, for instance, an object to be observed cannot be observed because of a profile, a size or the like of the object, the arrangement according to Document 1 cannot conveniently change NA of the objective lens unless the objective lens is replaced.